PPG 300 Modular

Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years. Whilst the Behringer modules cover most of the Moog modules I need, there are a few additional modules and changes I have made.

Big Moog with 303’s

PPG 303 LPF Klaus added a full set of PPG 300 analog modules and two PPG analog sequencers in 1976, and placed them on top of the Big Moog, where the ARP 2600 and EMS 8 Octave Bank usually lived. The PPG setup replaced the Synthanorma SQ312 analog sequencer, and his ARP 2600 in the live rig. He kept the PPG Modular setup until he moved onto Digital synths in the early 1980’s, and seems to have sold them quickly, whilst the Big Moog survived in his studio until 1996.

The Moog IIIP only has a few filters, and you can see the 904C set to Band Reject, so I am sure Klaus enjoyed the additional PPG sounds. He expanded his live rig into the late 1970’s with two Minimoog’s and a Multi Moog, so he loved the Moog filter. In 1980 Klaus swapped out the 991 and 994 and put two PPG 303 VCF/VCA modules into his Big Moog.

PPG 303 Artwork

AMSynths PPG 303 There are two types of PPG modular filter; the 303 VC Modifier with a 24dB transistor ladder filter (and a VCA) and the 317 VC Filter, which looks like a combined 904A/B/C. There are no schematics, so I have assumed the LPF is the same as in the PPG 1020, which sounds amazing! It is very similar in design to the traditional ladder filter as used in the Minimoog.

The AMSynths PPG 303 module is a replica of the VCF side of the 303, with the addition of input and output buffering Op Amps, to ensure I get the right voltages into the core. The filter uses 33nF 1% polypropylene capacitors in the ladder, as a replacement for the 1970’s Tropical Fish capacitors that were popular at the time. The panel is 8 HP wide and printed in black and white.

I built a prototype in September 2020, which after a few corrections worked well, except for the resonance which gave very strange results. I tried a few changes and in the end reverted to using the post Op Amp audio signal for regeneration, with the level controlled via a 100K potentiometer, like the ARP 4035. This works brilliantly, producing amazing resonance and self oscillation.

I set the gain structure so that hotter signals from the CP3A-O will overload the ladder filter and create some distorted overdrive. The CV inputs are set for the Behringer/Moog +6V outputs. The POTS PCB was revised to a version 2 to take account of the revised feedback circuit, and production PCB’s ordered and built.

Outcomes This filter sounds very good! A typical Minimoog sound with deep bass and clear sparkling resonance into self oscillation, with the option of a little distortion. The matched polypropylene capacitors have helped push the sound to a new level, and it is a more involving sound than many other filters. Many happy hours playing with this module!

Postscript In February 2021 I was able to see some photos of the inside of a PPG 317, which confirmed it was two PPG 104 ladder filters bolted together. The filter uses 3x CA3086 chips as expected. The HPF is the same PCB as the LPF but with minor changes, and there is a third PCB (with Op Amp) doing the BPF tricks, along with a 2P5T rotary switch. The ladder filter and buffer capacitors are 33nF and 150nF Wima polypropylene, with a 180uF 35V base ladder electrolytic capacitor.

So my replica is pretty close and its tempting to make the whole 317. I will switch out the 470uF cap to a nice ELNA 180uF, and the next board revision will take MKP4 buffer caps. I cannot trace the whole 317 schematic as there is not enough photos with the detail, but I will check how the HPF was created and how a V/OCT trimmer was used.






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